1. Field of the Invention
The invention relates to particulate material metering, and more particularly, to metering devices and methods wherein the material is fluidized and aerated to an uncompacted state prior to volumetric metering to achieve a high degree of accuracy over a wide range of masses.
2. History of the Prior Art
Most particulate materials are heterogeneous in nature in that they are comprised of both larger particles and more finely divided, powdery particles in an approximately randomly distributed mixture. Because of this random distribution, however, mass of particulate material also has homogeneous aspects in that two randomly selected equal volumes of uncompacted material should include essentially the same mixture of coarser particles and more finely divided material, so that the same mass of material is present in the two volumes. Two major problems occur in volumetrically metering preselected masses of particulate material. First, the material must be placed in a state of uniform compactness so that successive volumetric units metered from a quantity of the material will contain the same mass. Second, the material must be handled prior to volumetric metering in a manner so that the random distribution of coarser and more finely divided material is not disturbed; that is, so that the powdery and granular constituents of the material are not stratified and separated out from one another.
Many prior art metering and dispensing devices have employed various frequencies and amplitudes of vibration in order to feed material from a hopper to a dispensing mechanism. However, a major disadvantage of the use of vibration is that, while rendering masses more able to flow it also causes the particulate material, composed of different sized grains, to suffer stratification. That is, the more finely divided material is lost from the upper layers of material to the lower layers resulting in a concentration of the larger particles in the upper layers. Because of the greater bulk density of small particles of a given material than the larger particles, vibration results in cyclical variations in the rate of dispensing from a container loaded by several serially added batches of material.
Other prior art metering and dispensing devices have employed rotating arms which serve either as spinners, for moving material by centrifugal force, or as movable compartments for confining a particular volume of material between adjacent ones of the arms and then dispensing the confined material. While the present system employs a moving distributor blade it is rotated too slowly to generate any appreciable centrifugal force. Rather the blade is used in a unique fashion to fluidize the material and generate a slowly moving mound of material having fixed geometric characteristics from which a predetermined mass of material is metered.
For certain applications, for example, the metering and mixing of microingredients such as growth stimulants, antibiotics, amino acids, trace minerals and vitamins into animal feed, accuracy is very important. One of the most accurate prior art metering instruments is weigh belts wherein material is dispensed onto a movable belt supported by electrical weight sensors. Other relatively accurate prior art instruments include certain low rate feeders wherein material is dispensed into a groove of known volume cut in the surface of a disc. The material is then struck off level with the surface of the disc leaving a known mass of the material within the groove. In these prior art devices, however, a high degree of accuracy is obtainable only over a very small range of values. The system of the present invention is capable of dispensing particulate material with a high degree of accuracy over a wide range of values.